Gas turbine engines include compressor rotors having a plurality of rotating compressor blades. Minimizing the leakage of air, such as between tips of rotating blades and a casing of the gas turbine engine, increases the efficiency of the gas turbine engine because the leakage of air over the tips of the blades can cause aerodynamic efficiency losses. To minimize this, the gap at tips of the blades is set small and at certain conditions, the blade tips may rub against and engage an abradable seal at the casing of the gas turbine. The abradability of the seal material prevents damage to the blades while the seal material itself wears to generate an optimized mating surface and thus reduce the leakage of air.
Abradable seals have also been used in turbines to reduce the gap between a rotor and a vane. Thermally sprayed abradable seals have been used in gas turbine engines since the late 1960s. The seals have been made as coatings from porous and composite materials that derive their abradability from the use of low shear strength materials or from a porous, friable coating.
Prior art porous metal seals are commonly produced by thermally co-depositing metal seal particles and fugitive polymer particles. Heat treating the seal coating following deposition causes the polymer particles to decompose leaving a porous microstructure. The decomposition products vent through interconnected porosity during the heat treatment resulting in a lower density abradable seal coating. The gaseous permeability of prior art porous seal coatings can contribute to leakage bypassing the air foils and reducing engine efficiency.